De Anima II. 1 (3)

If the eye were an animal, vision would be its soul. But when the eye is a part of the animal, it can lose this ability without ceasing to be alive. This is the difference between the parts of living beings and the parts of machines, which latter are called dead iff they lose the ability to function.

And so we can make the account of soul that Aristotle is driving at more explicit as that which gives the parts of a complex physical system an existence beyond their abilities to function.

The existence in question is ultimately the self or individual which, if the definition holds, we have no reason to posit in complex physical systems that are not alive. And so soul is also the principle of individuation in the sense of being a self or individual as opposed to individuation as enumerated and therefore homogeneous quantities. Note that even a being that we imagine passing the Turing test would not be a self in this sense, even if they have a perfect performative or functional ability to mimic what is most distinctive in human behavior (do we really need to point this out? Is there any doubt that the there is more to being a dog than mimicking what is most distinctive about canine activity?)